1. Field of the Invention
The present invention generally relates to image processing, and more particularly relates to a white balance method and a white balance device for carrying out white balance with regard to an image.
2. Description of the Related Art
In image processing, it is often necessary to carry out white balance processing and/or color enhancement processing with regard to an image.
In particular, in a case of capturing a whiteboard image, it is often necessary to carry out color correction processing such as white balance correction with regard to the whiteboard image.
Up to now, a few technologies of white balance processing have been proposed. In the below cited reference No. 1, a technology of whiteboard image processing including region recognition, white balance, etc., is proposed. The white balance method includes a step of dividing a whiteboard image into plural small blocks; a step of ranking brightness of pixels forming each of the plural small blocks; a step of calculating or presuming background color of each of the plural small blocks so as to make up background color of the whole whiteboard image; and a step of carrying out the white balance by using the background color of the whole whiteboard image.
In the below cited reference No. 2, an algorithm of carrying out white balance by seeking gray color points in a YUV color space is described. In the YUV color space, the gray color points are found based on a condition of
      F    ⁡          (              Y        ,        U        ,        V            )        =                    (                                          U                                +                                  V                                      )            Y        <          T      .      After the gray color points are found, it is possible to obtain color deviation information, and then the white balance is carried out according to the color deviation information.
In addition, in the below cited reference No. 3, a method of carrying out white balance by using a human face is disclosed. In general, skin color of human beings is within a predetermined color range. As a result, by seeking the human face according to human face recognition, it is possible to acquire the skin color of the human face. After acquiring the skin color of the human face, the white balance is carried out by using the color deviation between the skin color of the human face and existing data of skin color.
In an image captured by, for example, a whiteboard sharing system (i.e., a system by which plural meeting rooms may share a same whiteboard remotely), the color of the whiteboard has deviation due to, for example, inaccurate white balance of a camera head. As result, it is often necessary to carry out white balance processing with regard to the image captured by the camera head.
Furthermore, in a conventional white balance system, color correction is carried out with regard to all pixels of an image by adopting a unified color correction parameter in general.
However, the inventors of the present invention found that, for example, under a complicated lighting condition, a captured image (for example, a whiteboard image) often has the following characteristics: as shown in FIG. 1, some portions are brighter; some portions are darker; color tone of some portions tends to be warm; and color tone of some portions tends to be cool. At this time, if only one global color correction parameter is adopted to carry out the color correction with regard to the captured image, then it may be impossible to better correct color deviation, and impossible to obtain a whiteboard image approaching that of a real whiteboard.
Cited Reference No. 1: Zhengyou Zhang and Li-wei He, Whiteboard Scanning and Image Enhancement, Microsoft Research, Technical Report MSR-TR-2003-39, June 2003
Cited Reference No. 2: Jun-yan Huo, Yi-lin Chang, Jing Wang, and Xiao-xia Wei, “Robust Automatic White Balance Algorithm using Gray Color Points in Images”, IEEE Transactions on Consumer Electronics, Vol. 52, No. 2, Pages 541-546, May 2006
Cited Reference No. 3: US Patent Application Publication NO. 2004/0208363 A1